perf(ui): a next-page fetch joins the one in flight instead of sending the cursor again

react-query's fetchNextPage cancels a next page still loading and sends it again by default.
Our requests ignore the abort signal, so the server ran both: the feed's read-ahead effect
re-runs on scroll frames before isFetchingNextPage reaches it, and on prod every feed page
went out 2-3 times (a 0.8 s title sort queued to 2.3 s). Every call now passes NEXT_PAGE
({cancelRefetch: false}), lint-enforced, with a contract test.

frontend/src/components/VirtualFeed.tsx

@@ -1,5 +1,6 @@
 import type { Video, VideoStatus } from "../lib/api";
 import { FEED_VIEW_SPEC, type FeedView } from "../lib/feedView";
+import type { NEXT_PAGE } from "../lib/nextPage";
 import VideoCard from "./VideoCard";
 import { VideoMenu } from "./VideoMenu";
 import { useContextMenu } from "./ui/useContextMenu";
@@ -29,7 +30,7 @@ export interface VirtualFeedProps {
   onOpen: (v: Video, startAt?: number | null) => void;
   hasNextPage: boolean;
   isFetchingNextPage: boolean;
-  fetchNextPage: () => void;
+  fetchNextPage: (options: typeof NEXT_PAGE) => unknown;
 }
 
 // The feed's videos on the shared VirtualGrid engine — this wrapper only maps a FeedView to the